[Show abstract][Hide abstract]ABSTRACT: We report pump-probe experiments employing laser-synchronized reactions of parahydrogen (para-H2) with transition metal dihydride complexes in conjunction with nuclear magnetic resonance (NMR) detection. The pump-probe experiment consists of a single nanosecond laser pump pulse followed, after a precisely defined delay, by a single rf probe pulse. Laser irradiation eliminates H2 from either Ru(PPh3)3(CO)(H)2 1 or cis-Ru(dppe)2(H)2 2 in C6D6 solution. Reaction with para-H2 then regenerates 1 and 2 in a well-defined nuclear spin state that exhibits a magnetic coherence in the plane orthogonal to the magnetic field without rf initialization. Detection of this coherence through its evolution is possible just 10 μs after the laser pulse and incrementation of the pump-probe delay fully-maps its behavior on a microsecond-millisecond timescale. The resulting high-resolution, single-scan NMR spectra can have hydride signal-to-noise ratios exceeding 750:1. The spectra for 1 oscillate in intensity with frequency 1101 ± 3 Hz, the chemical shift difference between the chemically inequivalent hydrides. The corresponding hydride signals of 2 oscillate with frequency 83 ± 5 Hz which matches the difference between couplings of the hydrides to the equatorial 31P nuclei. We use the product operator formalism to account for this oscillatory behavior and demonstrate how chemical shift imaging can differentiate the region of laser irradiation thereby differentiating thermal and photochemical reactivity within the NMR tube.

[Show abstract][Hide abstract]ABSTRACT: The synthesis, characterisation and thermal and photochemical reactivity of Ru(CO)2(PPh3)(dppe) 1 towards hydrogen are described. Compound proved to exist in both fac (major) and mer forms in solution. Under thermal conditions, PPh3 is lost from 1 in the major reaction pathway and the known complex Ru(CO)2(dppe)(H)2 2 is formed. Photochemically, CO loss is the dominant process, leading to the alternative dihydride Ru(CO)(PPh3)(dppe)(H)2 3. The major isomer of 3, viz. 3a, contains hydride ligands that are trans to CO and trans to one of the phosphorus atoms of the dppe ligand but a second isomer, 3b, where both hydride ligands are trans to distinct phosphines, is also formed. On the NMR timescale, no interconversion of 3a and 3b was observed, although hydride site interchange is evident with activation parameters of DeltaH(double dagger) = 95 +/- 6 kJ mol(-1) and DeltaS(double dagger) = 26 +/- 17 J K(-1) mol(-1). Density functional theory confirms that the observed species are the most stable isomeric forms, and suggests that hydride exchange occurs via a transition state featuring an eta2-coordinated H2 unit.

[Show abstract][Hide abstract]ABSTRACT: We describe a number of studies used to establish that parahydrogen can be used to prepare a two-spin system in a pure state, which is suitable for implementing NMR quantum computation. States are generated by pulsed and continuous-wave (CW) UV laser initiation of a chemical reaction between Ru(CO)(3)(L(2)) [where L(2) = dppe = 1,2-bis(diphenylphosphino)ethane or L(2) = dpae = 1,2-bis(diphenylarsino)ethane] with pure parahydrogen (generated at 18 K). This process forms Ru(CO)(2)(dppe)(H)(2) and Ru(CO)(2)(dpae)(H)(2) on a sub-microsecond time-scale. With the pulsed laser, the spin state of the hydride nuclei in Ru(CO)(2)(dppe)(H)(2) has a purity of 89.8 +/- 2.6% (from 12 measurements). To achieve comparable results by cooling would require a temperature of 6.6 mK, which is unmanageable in the liquid state, or an impractical magnetic field of 0.44 MT at room temperature. In the case of CW initiation, reduced state purities are observed due to natural signal relaxation even when a spin-lock is used to prevent dephasing. When Ru(CO)(3)(dpae) and pulsed laser excitation are utilized, the corresponding dihydride product spin state purity was determined as 106 +/- 4% of the theoretical maximum. In other words, the state prepared using Ru(CO)(3)(dpae) as the precursor is indistinguishable from a pure state.

[Show abstract][Hide abstract]ABSTRACT: The photochemical reaction of Ru(CO)(3)(L)(2), where L = PPh(3), PMe(3), PCy(3) and P(p-tolyl)(3) with parahydrogen (p-H(2)) has been studied by in-situ NMR spectroscopy and shown to result in two competing processes. The first of these involves loss of CO and results in the formation of the cis-cis-trans-L isomer of Ru(CO)(2)(L)(2)(H)(2), while in the second, a single photon induces loss of both CO and L and leads to the formation of cis-cis-cis Ru(CO)(2)(L)(2)(H)(2) and Ru(CO)(2)(L)(solvent)(H)(2) where solvent = toluene, THF and pyridine (py). In the case of L = PPh(3), cis-cis-trans-L Ru(CO)(2)(L)(2)(H)(2) is shown to be an effective hydrogenation catalyst with rate limiting phosphine dissociation proceeding at a rate of 2.2 s(-1) in pyridine at 355 K. Theoretical calculations and experimental observations show that H(2) addition to the Ru(CO)(2)(L)(2) proceeds to form cis-cis-trans-L Ru(CO)(2)(L)(2)(H)(2) as the major product via addition over the pi-accepting OC-Ru-CO axis.

[Show abstract][Hide abstract]ABSTRACT: The cover picture shows the reaction pathways available to Ru3(CO)10(dppe) in the catalytic hydrogenation of diphyenylacetylene, as elucidated by parahydrogen-assised NMR spectroscopy. The cluster leads to catalytic hydrogenation via a detected vinyl hydride intermediate, but fragments in low polarity solvents. Details are discussed in the article by S. B. Duckett et al. on p. 4381 ff. We thank Universities UK (ORS award), Bruker Biospin UK and The University of York for financial support.

[Show abstract][Hide abstract]ABSTRACT: Twirl operations, which convert impure singlet states into Werner states, play an important role in many schemes for entanglement purification. In this paper we describe strategies for implementing twirl operations, with an emphasis on methods suitable for ensemble quantum information processors such as nuclear magnetic resonance (NMR) quantum computers. We implement our twirl operation on a general two-spin mixed state using liquid state NMR techniques, demonstrating that we can obtain the singlet Werner state with high fidelity.

[Show abstract][Hide abstract]ABSTRACT: The study of reaction mechanisms by NMR spectroscopy normally suffers from limitations in sensitivity that arise from the physical constraints of the detection method. An overview is presented of how chemical reactions can be studied using parahydrogen assisted NMR spectroscopy where detected signal strengths can exceed those normally seen by factors of over 28,000.

[Show abstract][Hide abstract]ABSTRACT: We demonstrate the implementation of a quantum algorithm on a liquid-state NMR quantum computer using almost pure states. This was achieved using a two-qubit device where the initial state is an almost pure singlet nuclear spin state of a pair of 1H nuclei arising from a chemical reaction involving parahydrogen. We have implemented Deutsch’s algorithm for distinguishing between constant and balanced functions with a single query.

[Show abstract][Hide abstract]ABSTRACT: We demonstrate the implementation of Grover’s quantum search algorithm on a liquid state nuclear magnetic resonance quantum computer using essentially pure states. This was achieved using a two qubit device where the initial state is an essentially pure (ε = 1.06 ± 0.04) singlet nuclear spin state of a pair of 1H nuclei arising from a chemical reaction involving para-hydrogen. We have implemented Grover’s search to find one of four inputs which satisfies a function.

[Show abstract][Hide abstract]ABSTRACT: The phosphido-substituted triruthenium cluster Ru(3)(CO)(9)(mu-H)(micro-PPh(2)) is shown to react with H(2) to form the trihydride cluster Ru(3)(CO)(9)(H)(mu-H)(2)(mu-PPh(2)), which undergoes a number of re-arrangement reactions on heating to yield other phosphido-substituted triruthenium clusters. In the presence of alkyne substrates, heating the system leads to catalytic hydrogenation via CO loss and the formation of a Ru(3)(eta(2)-PhC[double bond, length as m-dash]CHPh)(CO)(8)(micro-H)(PHPh(2)) resting state, in a reaction affected by the polarity of the solvent. No mononuclear fragments are observed in the catalytic transformation, confirming directly that the phosphido ligand is able to exert a stabilising influence on the cluster core.

[Show abstract][Hide abstract]ABSTRACT: Here we demonstrate how parahydrogen can be used to prepare a two-spin system in an almost pure state which is suitable for implementing nuclear magnetic resonance quantum computation. A 12 ns laser pulse is used to initiate a chemical reaction involving pure parahydrogen (the nuclear spin singlet of H2). The product, formed on the micros time scale, contains a hydrogen-derived two-spin system with an effective spin-state purity of 0.916. To achieve a comparable result by direct cooling would require an unmanageable (in the liquid state) temperature of 6.4 mK or an impractical magnetic field of 0.45 MT at room temperature. The resulting spin state has an entanglement of formation of 0.822 and cannot be described by local hidden variable models.

[Show abstract][Hide abstract]ABSTRACT: The reaction of Pd(cod)Cl-2 (cod = cycloocta-1,5-diene) with 1 and 2 equiv. of rac-diphenyl[2.2]paracyclophanylphosphine, rac-PPh2(C16H15), affords the dimer [Pd{PPh2(C16H15)}Cl-2](2) (1) and the square-planar complex trans-Pd{PPh2(C16H15)}(2)Cl-2 (2), respectively. In solution the dimer undergoes a fluxional process which has been probed by NMR and involves isomerisation between pseudo-trans- and cis-conformations. The structures of trans -[Pd{Ph-2(C16H15)}Cl-2](2) (1a) and 2 have been established by single crystal X-ray diffraction; the structure of the dimer is severely disordered. In addition, co-crystals containing both these complexes and solvate molecules have been isolated and their structures established by single crystal X-ray diffraction. The structure of the monomer in the homonuclear and co-crystals are not too dissimilar whereas the structure the dimer has a significantly different structure in the homonuclear and co-crystals. In the homonuclear crystal the central Pd2Cl2 unit has a dihedral angle of 26.5degrees between the two planes whereas the Pd2Cl2 unit in the co-crystals is planar. (C) 2003 Elsevier B.V. All rights reserved.

[Show abstract][Hide abstract]ABSTRACT: The reactivity of the cluster family [Ru(3)(CO)(12-x)(L)(x)] (in which L=PMe(3), PMe(2)Ph, PPh(3) and PCy(3), x=1-3) towards hydrogen is described. When x=2, three isomers of [Ru(3)(H)(mu-H)(CO)(9)(L)(2)] are formed, which differ in the arrangement of their equatorial phosphines. Kinetic studies reveal the presence of intra- and inter-isomer exchange processes with activation parameters and solvent effects indicating the involvement of ruthenium-ruthenium bond heterolysis and CO loss, respectively. When x=3, reaction with H(2) proceeds to form identical products to those found with x=2, while when x=1 a single isomer of [Ru(3)(H)(mu-H)(CO)(10)(L)] is formed. Species [Ru(3)(H)(mu-H)(CO)(9)(L)(2)] have been shown to play a kinetically significant role in the hydrogenation of an alkyne substrate through initial CO loss, with rates of H(2) transfer being explicitly determined for each isomer. A less significant secondary reaction involving loss of L yields a detectable product that contains both a pendant vinyl unit and a bridging hydride ligand. Competing pathways that involve fragmentation to form [Ru(H)(2)(CO)(2)(L)(alkyne)] are also observed and shown to be favoured by nonpolar solvents. Kinetic data reveal that catalysis based on [Ru(3)(CO)(10)(PPh(3))(2)] is the most efficient although [Ru(3)(H)(mu-H)(CO)(9)(PMe(3))(2)] corresponds to the most active of the detected intermediates.

[Show abstract][Hide abstract]ABSTRACT: The clusters Ru(3)(CO)(10)L(2), where L = PMe(2)Ph or PPh(3), are shown by NMR spectroscopy to exist in solution in at least three isomeric forms, one with both phosphines in the equatorial plane on the same ruthenium center and the others with phosphines in the equatorial plane on different ruthenium centers. Isomer interconversion for Ru(3)(CO)(10)(PMe(2)Ph)(2) is highly solvent dependent, with DeltaH decreasing and DeltaS becoming more negative as the polarity of the solvent increases. The stabilities of the isomers and their rates of interconversion depend on the phosphine ligand. A mechanism that accounts for isomer interchange involving Ru-Ru bond heterolysis is suggested. The products of the reaction of Ru(3)(CO)(10)L(2) with hydrogen have been monitored by NMR spectroscopy via normal and para hydrogen-enhanced methods. Two hydrogen addition products are observed with each containing one bridging and one terminal hydride ligand. EXSY spectroscopy reveals that both intra- and interisomer hydride exchange occurs on the NMR time scale. On the basis of the evidence available, mechanisms for hydride interchange involving Ru-Ru bond heterolysis and CO loss are proposed.